When your current print is done, wrap some packing tape tightly around the portion of the spool that looks like it’s wound properly to prevent it from becoming any more unwound. Then you’ll have to manually unwind the tangled portion until you get to the properly wound portion. Now it gets a bit tricky. With one hand CONSTANTLY holding the filament where it comes off the roll and keeping the filament tight on the spool, remove the packing tape holding the properly wound filament in place. Then, while keeping tension on the spool and that filament still passing through your hand, wind the spool back up. Try to keep the filament neat and tight as you wind it back up. If a tangle starts to form on the other side of the hand that’s holding the filament, DO NOT LET GO, but keep the tension on the spool and use your free hand to clear up the tangle and continue winding. It’s tedious.
The most common cause is that you let go of the filament end when preparing to mount it and the filament uncoiled a bit and tied itself up. These spools are wound by machine and generally can never get tangled unless handled. That having been said, there is one way that it can get knotted even if the end of the filament is held firmly, but it involves at least two loops of filament slipping off the spool but going back on in reverse order.