Does it look sad or did the color change to blue?
Bike shops generally do not have electricians, and you need an electrician.
At that age, you most likely need to replace all the cells, depending on how you have used it, stored it, and charged it, and since you have provided virtually no useful diagnostic information, that is the most likely case.
If you provided measured pack voltage after a full charge, again after sitting for a few days, then charged full and rode 1/2 of your range when it was new and then tested again, or just told us the range when new in miles, and the range today, then maybe I could offer something better than guesswork.
Re-celling older batteries is generally not cost-effective, unless it is a proprietary design that forces you to use the manufacturer's replacement parts at exorbitant costs. Just buy a new battery.
There are a very, VERY small number of battery kits that allow you to replace cells yourself and keep the BMS, case, wiring, etc. Actually only one that is commercially available that I know of. N.E.S.E. Not that Vruzend crap.
