ive taken the coolant reservoir cap off my v6 jag & can not get it back on any t

No certain way. It screws off and screws back on. It is possible that the seal has expanded a little and preventing you from seating it down far enough for the threads to engage. You may need to press down on the cap firmly, then try and twist to engage the threads. If that doesn't help, you may need to replace the cap.