Battery problems?

I Have a macbook Pro early 2011 that the battery life is quickly declining on. I can't get through 3 hours without it shutting down bc the battery died. Today it died and wouldn't turnback on. I opened it up and unplugged the battery and plugged it back in. It started after that. when battery runs out, the computer never runs well once I plug it back in and needs a restart. It may just be MacPractice issue, but being that's what I'm using primarily, it requires a restart. Any ideas why the battery dies so fast? Why would the computer not turn on? Does unplugging the battery inside the computer do something that it would turn on after I did that and put the back of the computer back on?