If you have sliding tub or shower doors that are stuck, you should first focus your attention at the top of the doors.
If you are looking to just open the doors to service the rollers (likely the root cause of the problem), you should be able to lift the door slightly and gently move it down the head rail. That will at least allow you to enter the tub/shower to fix the problem.
You might get lucky and find that one of the wheel assemblies is no longer riding on the track. This fix will be simply lifting that side of the door up and placing it so that the wheels are aligned with the track and it should roll smoothly.
It may also be that the wheel assembly has come loose and needs to be adjusted. You will need to remove the door to get access to the screws that hold these in place. Lift up the door and swing the bottom out. Then you can clear the top of the door from the head rail. The vinyl wheels don’t normally get ruined because the door isn’t that heavy. However, if they look bad, now is a good time to replace them.
Adjust the wheel assembly so that it is in the middle of the adjustment range. Rehang the door and see if it rolls smoothly. If they don’t adjust the wheels downward to lift the door slightly higher from the bottom track.