It could because of the first gear is worn out. It would be a good idea to have it looked at by certified mechanic.
Sensors got wet
There could be a transmission fluid leak. Check the seals. They may be loose or worn out.
Well, that would depend on the transmission. Sounds like you got yerself a 2wd mf-er.
The shift censor could be going bad. There may also be a problem with the transmission linkage which would cause this problem.
You need to replace your clutch.
If it is an automatic transmission then you would have to remove the transmission fuid pan and it is the first thing you will see in there. I would also recommend that you drain the fuid first... but I would also recommend that you take it to a trained professional to do. You need to keep the inner workings of your transmission clean.
Most likely, the fluid is low, or the filter is dirty.
These are unrelated failures. The transmission has a fail safe system in that a electrical or electronic failure will still allow the car to move both forward and back. So the transmission failure is a mechanical one inside the transmission and unrelated to the alternator problem
VSS (Vehicle Speed Sensor) failure.
Stripped transmission, low transmission fluid.
IF YOUR TALKING ABOUT THE TRANSMISSION , ITS SLIPPING AND IT SOUND LIKE YOU EITHER HAVE NOT BEEN PUTTING TRANSMISSION FLUID IN THE CAR ON A REGULAR BASIS OR YOU HAVE MESSED UP THE TRANSMISSION ALL TOGETHER YOU NEED TO TAKE TO A TRAMISSION SHOP AND LET THEM DO A DIAGNOSTIC CHECK ON THE CAR, MORE AND LIKELY YOU NEED A TRANSMISSION REPLACEMENT. OR A COMPONENT REPLACE ON THE TRANS.
There are several things that can cause your automatic transmission not to go into first gear. The most common cause is low transmission fluid.