Take your ball into the pro shop and have him check the fit. You would be surprised how much your hand can change taking a couple months off. Also if you have finger inserts in your ball, make sure they feel good and fit properly. Beginning of league season is always a good time to start out with a fresh set of finger inserts.
Have your pro shop professional inspect the surface of the ball. More than likely as you got towards the end of last season, ball maintenance was lacking. Now is a good time to have your ball professionally cleaned or resurfaced.
The summer months are also when manufacturers introduce new bowling balls and products to the pro shops to be ready for the winter season. Check with your pro shop professional to see if there is something new that might fit your game better than what you currently have. If it’s been a couple years since drilling a new ball, now might be the time.
Even if your equipment is in good working order and fitting properly, remember you took the summer off. Take your time when warming up. Stretch a little. Don’t go too fast. Especially that first week. Just work on a nice smooth tempo. If after a few weeks things still aren’t coming together, maybe it’s time to find a qualified bowling instructor. And it may only take one session. Having a trained eye to help you and create a practice plan can be extremely beneficial.