Well, after such a long time I am not sure anymore if it will still charge properly. Have a look at this topic:
Generally, these batteries degrade over time. How old is your battery? And if you don’t know yet, letting the battery run all empty repeatedly will damage it. Depending on how your battery fared in these respects so far, it might just be worn out.
If you know someone with a FP2, you might want to check if their battery works in your FP2. Or if there is a Fairphone Angel nearby, s/he might allow you to check that: