Hi Jack!
Well, could be a lot of things.
The dock won't recognize it? Could be the USB-C port...
What I always do, is temporarily use a fully charged (new) battery and see if it want to turn on.
If the Switch does turn on, then put the charger in. If it is charging, then it's probably the battery which need to be replaced. If it won't charge, then you might want to replace the USB-C port. If it's still not working after replacing the USB-C port, then it's motherboard related.
So I recommend to try a fully charged new battery first and see what happens. This will help you troubleshooting.