Need help asap-can't switch gears while driving
 

My 98 5 speed maxima won't let me switch gears while I'm driving. What could be the issue?

Check your hydraulics out.
Slave, master, or even the line for a leak.
Master will leak in cab and the slave will leak on top of trans.
If your fluids are full and you don't have air in the system it may be time for a new clutch or there may be a bigger problem.

Easiest place to start would be hydraulics before jumping to other conclusions.

Do you still have the same feel to your pedal as usual?
Do you use GL-4 gear oil or GL-5?