MY GMC TH700 auto trans won't shift out of second gear?

[TIMBOSLICED]

I did a filter change on my S15 pick-up. after I buttoned every thing back up. I took it out for a 50 mile test drive and had no problems. this morning I get up and it won't shift out of second gear. The fluid level is normal. although it says check hot. but I have no leaks that I can see. The pan seal area is dry. Why did it work fine yesterday and not today ? I only went about a half a mile today when it didn't shift and went back home to scratch my head the rest of the day. What is going on ? Did I not give it a chance to shift or what ?