HELP! I've not fallen, but I can't get up!!

Background:  I'm 78 years old.  After living 30 years on a lake with the last 7 of them course skiing, I lived and cruised 15 (2007-2022) years on a sailboat. I climbed the 140-ft/220 steps St. Augustine Lighthouse with no knee pain 2 years ago,and other greater or lesser ones earlier in that time.  I've now lived 18 months ashore.  I believe I'm reasonably fit. 

Today I have notable patellofemoral pain on knee effort.  Standing from a chair, e.g., or squats, or doing seated leg curls forward is very painful. (Reverse - pulling - is very much less so.)  That is the only segment of either upper or lower body work I do 4x weekly at gym, now for a year (living on a cruising sailboat was a great way to stay in general good shape!), where I have any pain.

So, last week, behind a Nautique, and on a not-my-ski (my son can't find it in the garbage dump of his basement, but had a wider 69" HO Charger with widely adjustable full boots which - with much soap - accepted my feet), I go in for my first pull.  The last regular-boat pull (~2010?) many years ago, after many years without skiing, I had needed a more typical rolling throttle start, so that's what I asked for this time.  I got out of the water but could not get up.  No issues with grip; my knees pushed hard enough against my chest that nearly a week later, I still can't sniff or cough without pain.  But my left buttock got blasted by the water torrent as I failed to get up off the ski.  On the possibility that I just didn't  have the strength on a smaller ski, I tried a pair.  Essentially the same result.

Yet, for a week I was coaching little kids and newbie adults, and remembered some of my mechanics that perhaps I didn't  do on that thwarted set of attempts.  Not rotating my hips adequately?  Inadequate back strength?  Or all down to the knee pain leading to lack of eventual leg strength?  All of the above?

I feel like I should be able to get up, and once I rediscover what I used to do effortlessly, should be able to ski many more years (one of the early VHS training tapes I and my son watched had an 86 year old driver for the camp doing the course, e.g.) on the lake to which I'll move in a year or so.

Ideas?  I likely won't get another chance to ski before my move, but also will have that time to work out and work on whatever portion of my mechanics which is failing.  And finally, has anyone had the same pain issues but has resolved them?

Thanks.