Most likely alternator has stopped charging the battery!! .. Look to see if the 'alternator belt' is still intact and tensioned properly, also possible fuse/wiring problem, the charging 'system' (inclu. battery) needs to be tested before alternator is condemned!!
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Okay all check the battery first and see if the belt is in place I hope this don't cost me a lot. Thank you.
U R Welcome... I hope it's a simple fix too!