Change of a 94 Monza

I own a Monza GL 1.8 EFI (original gasoline engine).
My problem is that the gearbox is malfunctioning.
Reverse gear won't engage at all, nor will second gear. First gear is currently stuck in reverse.
I need to know what's wrong and what the solution is. Can I fix it myself, or do I need a mechanic? Thanks in advance.

IS IT AUTOMATIC OR SYNCHRONOUS?

The transmission is synchronous. I need a clear explanation. If I can fix it myself, please tell me how, or if you have a workshop manual or anything that could help me. Thank you for now

First you will have
to tell us if that happened before we intervened or suddenly.

No, I haven't touched it yet.
I bought the car used not long ago, and it started having problems recently. I needed it for work, so I kept driving it. At first, all the gears worked fine. Occasionally, reverse wouldn't engage, but all the others worked more or less okay. Now they don't.
I like to ask questions first before doing anything myself. When you don't know something, it's best to ask and find out, then act. Thanks, I hope you can help me.