Filament stuck on spool
If the filament isn't wound properly around the spool, it can tighten itself into almost a "knot" and make it impossible for the extruder to pull it into the printer.
You can manually work the filament around the spool to try and loosen it. Unfortunately, short of rewinding the entire spool, there's not really a good way to fix this permanently. It's a good idea to keep a close eye on the printer for the remaining of the print to ensure that it doesn't become tangled again.
This is why it's super important to always thread the filament though the two holes on the spool holder after use, to ensure it doesn't become tangled!