There can be at least two reasons:
1) sw-engine is not running, that's the panel itself. Very unlikely though unless it was deliberately stopped. Solution: Login to SSH and run as root service sw-engine restart && service sw-cp-server restart
.
2) Fail2Ban has banned your IP for too many invalid login attempts. Solution: Try to access the panel through another IP, e.g. your cell phone when it is connected to a cell, not your wifi. From there remove your local public router IP from the bans.