LiPo batteries need to be recharged according to a certain protocol or they will be quickly destroyed.
They are also very quickly destroyed by overdischarge, and need protection circuitry. Some cells come with protection circuitry built in, and if you have one of those, it may need resetting, as it could be preventing you from charging the battery.
There are charger modules designed to work with solar cells, like this one: USB / DC / Solar Lithium Ion/Polymer charger [v2] : ID 390 : $17.50 : Adafruit Industries, Unique & fun DIY electronics and kits
You should do some research on the topic before proceeding.