Not that I am an expert, but I am guessing that there are really two simple possibilities. You either dislodged or misaligned the little magnet or piece that is located on a spoke your front wheel or dislodged or misaligned the part that is located on your fork. Either way, easy fix. Fnd that little part on the spoke and make sure that it is passing the reader (that is the part that sits on your fork and reads how many times/fast the piece on your spoke goes by) no further away than about 5 mm. They also need to be lined up so that the spoke piece is basically centered on the piece mounted on your fork.
If that sounds too confusing, I can probably get you a link to their site where they have pictures - sometimes I need pictures to help me figure it out!![]()