Hello and welcome to the forum.
Take a look at
this thread. It's from the 500 section, but the transmissions are identical.
Start by disconnecting the battery, and make sure the earthing between the battery and chassis is sound.
It's not that likely this will fix anything, but it costs nothing and can't hurt.
Next step is to get any stored fault codes read; this needs Fiat-specific software (most generic code readers can't interrogate transmission faults). If you give us an idea of your location, there may be someone local to you with the appropriate tool to do this.
Failing that, finding an independent Fiat specialist with knowledge of these transmissions is probably your next calling point. It's possible that resetting the fault codes and (if needed) topping up the selespeed fluid may get things working again, though any cheap fix is unlikely to be more than temporary.
If you go to a franchised dealer with a fault like this, the most likely outcome is you'll be charged anywhere between £100-£200 to be told the car isn't worth repairing.
Realistically the best thing you can probably do now is to go look for a replacement car. It's unlikely to be cost effective to properly fix a significant transmission fault on a 15yr old dualogic Panda.