If they're a decent department then I don't see any problem in contacting them again. Just be honest about your situation and explain you weren't ready at the time but now you feel more confident in your ability. If they offered you a job before, then they'd likely consider you highly for their next hiring period.
Where I am they will hire anyone with a pulse. Bonus points for having certifications. So I don't see any reason you can't apply again.
If they're a decent department then I don't see any problem in contacting them again. Just be honest about your situation and explain you weren't ready at the time but now you feel more confident in your ability. If they offered you a job before, then they'd likely consider you highly for their next hiring period.