Beta decks use the size of the cassette traveling in the tape elevator and tripping little levers here and there to tell the machine how large the tape is.
If the smaller of the tapes reset these settings (successfully??) and one or more of the levers and trip wires got stuck, the machine would not know how to handle a larger tape.
Put the little tape in and out several times and then try a largert tape again. That exercise may knock something loose.
If that fails, you have little choice but to open the machine. Put the smaller tape in again and what what happens. Then put the larger tape in and watch where it gets stuck.
It's also possible that one of the optical positioning sensors failed. That's means a serious repair trip. Have you tried turning the machine off and unplugging it? Count to ten and plug it all back in again.
Koz