Assuming you are talking about a 510 pen
like this and not a pod system type, on those types of battery units, the solid white light usually means that the battery unit detects that the resistance on the
tank's (on your case a cart's) coil is lower than that battery unit allows.
Assuming that it worked before and assuming that no cart for that type of battery unit would have too low a resistance because you type of vapers never pay attention to that stuff (from what I have seen on reddit) so all the carts and battery units are pretty much the same more or less, my guess is that your cart's coil is really gunked up and the battery unit is reading the the resistance lower than it actually is. Nothing you can do if that is the case except get a new cart.
But one thing you can try before you get a new cart is to clean the 510 connection really well on your battery unit as well as on your cart with rubbing alcohol (91% preferably but 70% will do) and see if that fixes your problem.