1. Go to the General -> Settings and please make sure that your WordPress Address (URL) and your Site Address (URL) are exactly equal.
If they are unequal then please, correct it. If they are, then need to go to step 2.
2. If you have just updated your WordPress and this happened, then it is some sort of conflict with the plugin which is probably not compatible with your WordPress anymore.
If your problem is gone by deactivating plugins, then just enable your plugins one by one till you find your culprit.