Hi @nicklasfre83112
After this length of time most probably the battery has become depleted beyond recovery and will need to be replaced.
Here's the [https://download.lenovo.com/pccbbs/mobiles_pdf/lenovo-tablet_hmm_en.pdf|hardware maintenance manual] for the tablet that may help.
Go to p.52 to view the procedure to remove/replace the battery.
Search online for ''01AV468'' or ''5B10W13915'' to find suppliers of the battery, that suit you best.