No first gear or reverse gear

That would be the Mazda GSM R5 transmission. They are well known for bent/broken shift forks, failing synchronizers and shift linkage issues.

Can you shift it through the rest of the gears and it works you just cannot get into first and reverse? Or is the shifter just jammed and will not move?

If it is the first one it is probably linkage related and might be that the bushing in the shift rod has failed and is not pulling the linkage far enough to engage those two gears. I would also check the shift stabilizer bar, it sets the distance from the shifter to the trans-axle, if it breaks (or the bushing in it wears) or comes loose you could lose the ability to shift into some gears.

After that it would be internal to the transmission and for that you want the book in front of you. Both Alldata and Mitchell have single vehicle subscriptions for around thirty dollars for a year of full access, that gives you the same information the shops use and shows the full disassembly of that transmission.