Hi @lillylou
Try leaving the charger connected for 2-3 hours to see if this kick starts the battery into charging again.
If it does let it charge to 100% i.e. wait for 4-5 hours on charge and check, before attempting to turn on the phone.
If it does fully charge then try turning it on and check.
Hopefully it will but if it doesn't start to charge, fully charge or turn on the first thing then to try is a battery replacement. Not having charged the battery for several months could have meant that it became depleted beyond recovery to work properly again.
Here's the ifixit Samsung Galaxy S6 Edge Battery Replacement guide that may help.
Search online for EB-8G925ABE to find suppliers of the replacement battery, that suit you best.
If doing this yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.
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