Alternator belt got off

My car is Honda Accord 1990. The charging system warning light was on while driving, so I checked the alternator belt. I found it already got off the pulley. I tried to put it back, but failed. Should the alternator belt be replaced? If I have to ask a mechanic to do it, how much will it cost?
哪位高手能够之点一二，将不胜感谢。