The tranny has gone into limp mode, probably because it lost electrical power or because the PCM detected a fault. In limp mode you will not have 1st and 4th gears and torque converter lockup will not work. Reverse will still work but in drive (D) it will only operate in 3rd gear. 2nd gear will work but only if you put the shifter in 2nd gear and it will also operate in 2nd gear if you put the shifter in 1st gear. Your tranny is powered on the same circuit as the O2 sensors and if an O2 sensor shorts out it will blow the tranny fuse. I suspect that washing the engine may have shorted an O2 sensor and blew the tranny fuse. I suggest you check your fuses. If you don't find the problem there then check the harness connectors at the tranny. Also if your tranny fluid is brown with a mix of black like you said, once you restore electrical power to the tranny I suggest you waste no time flushing that fluid and changing the tranny filter. But before you do the flush I strongly recommend you add a good quality transmission cleaner to the transmission and let it circulated thru the tranny for about 15 minutes before flushing. That will clean up the transmission and break down the sludge and varnish build up so that it will be removed during the flush. Many people make a great mistake by not adding a high quality cleaner to the tranny before flushing and then the sludge and varnish build up is dislodged soon after flushing and clogs the valve body. GUNK TransMedic Transmission Flush or something of equal quality will do a good transmission clean up and break down the sludge and varnish build up so that it will be removed during the flush. If you experience harsh upshifts after the flush then I suggest you add a bottle of GUNK TransMedic Automatic Transmission Treatment. That will coat and better lubricate the pump, gears, and valve body internals which will help to smooth shifts.
But before you do anything I suggest you check the tranny fuse or do whatever troubleshooting is necessary to restore elecrical power to the tranny and if you must drive it in the meantime I suggest you manually shift it into 2nd gear to get it moving and then shift to drive (D) so that it shifts into 3rd gear because starting out in drive (D) will be 3rd gear while in limp mode and the increased torque converter slippage by starting out in 3rd gear will generate a lot more heat than normal and the excessive heat generated can do serious internal damage.
*Michael, I would recommend a full tranny flush rather than just a pan drain. But even just a pan drain will be beneficial. But if you do just a pan drain then I suggest you don't use a tranny cleaner before the drain because the cleaner must be completely removed from the tranny. And as for the GUNK TransMedic Transmission Flush, I have only found the synthetic at the Advance Auto Parts store I use but if it is available in non-synthetic then either will probably be fine. But I would still choose the synthetic because I'm sure that would lube better while it cleans. But I do know that the GUNK TransMedic Automatic Transmission Treatment is available in both synthetic and non-synthetic. I have seen both of those at the parts store. The non-synthetic GUNK TransMedic Automatic Transmission Treatment will be fine if you do a DEXRON-III fill, which was the GM factory fill ATF on your 1994 tranny. But if you do a fill with the new DEXRON-VI synthetic ATF then you might consider the synthetic GUNK TransMedic Automatic Transmission Treatment after the flush and fill. The synthetic DEXRON-VI ATF replaced all previous DEXRON fluids in 2006 and GM says the synthetic DEXRON-VI is compatible with all previous non-synthetic DEXRON fluids. It does not hurt to mix them. But if it was me, I would opt for the DEXRON-III fill. It's much cheaper than DEXRON-VI and it was the original factory fill on your tranny. But if you experience harsh upshifts after the flush and fill I do think it would be best to use the synthetic GUNK TransMedic Automatic Transmission Treatment even if you do a DEXRON-III fill. It will blend perfectly with non-synthetic DEXRON-III and I'm sure it will help to improve lubrication. Keep in mind though that the DEXRON-III license was not renewed after 2006 and it is getting hard to find labeled as DEXRON-III so if you have trouble finding it then look for the DEX/MERC ATF label. Since the license expired that is how DEXRON-III is labeled now (DEX/MERC).