Hp15c, using STO throws error 3. Did I break it?

[Spencer]

I'm using my 1985 hp15c to solve a 2nd order runge kutta scheme. A system of equations describing ballistic flight of an arrow through air. The program is about 60 lines long and uses only about 7 registers. I tried to STO 5 in the programming screen and it gave an error 3. Earlier I tried doing an f Lbl B and it gave error 4. It's almost like every single register does not exist. But earlier I made a 74 line program using almost all the 20 registers and it performed great. But now it's up in the air if I can even program at all. It's like it's not happy I put it through 18000 flops. I paid 200usd for this gem and I'm really afraid I broke it. Someone please help!