Ok so I been training well all since hardcore since June,. and saw amazing gains over the summer in my time trials and road races and overall regular rides in generals. Still on training plan. However Did a cyclocross event today, using my mt bike. And i was more less dead last. At start of event. I was push my HRT in high zone like my normal race pace like 180's etc.
but i was riding slower then a turtle.
Felt very weak and unable to push myself. Guys who i can normally beat without thinking about it, practically kicked my ass today.
Getting lapped by everyone .

What factors would cause this. I did show up there after a night shift though, so only on 3 hours sleep, so that could be a cause. however i have particpated before in races after getting off a night shift of working and being fully energized.

Thoughts please. Suggestions?
Is this a cause of overtraining? should i take time off the bike for some time? suggestions please? advice

Overtraining is the usual cause, but then HR is usually depressed during the ride, and elevated first thing in the morning, so that's pretty easy to spot. Another thing is hydration state. Being dehydrated can cause a high HR and slow times both. Normally having low blood sugar/glycogen causes a lowered HR.

You could get an inexpensive blood glucose meter, the kind meant for diabetics, to get a more informed read on your blood sugar levels. I once took one on a long ride and learned that I didn't need to eat nearly as much as I'd always thought.

Carbon is probably right, but if it persists, think about getting full bloodwork at a real dr. to see if you're out of range on any key nutrients or other markers.

Reminds me of a story. I did a century where I also rode to and from the start. It was a fairly competitive group ride, but after it I still had to get home. A diabetic riding buddy offered to ride home with me. Good friend. On the way, I really felt like I was not OK, so we stopped and did a blood test. Sugar was 100, a little low but fine. I was just pooped. I sucked it up and made it home.

The human engine is not really an engine as we think of mechanical devices. Machines exhibit similar performance day after day with some degradation due to wear and need of maintenance. There is not fatigue or emotional factor in mechanical systems and mechanical systems do not get sick.

Human performance just varies and it is due to many factors. My performance can vary a lot and many times it is mental and has nothing to do with physiology.

IMO, the most likely reason is that you had a bad day, not enough sleep that is due to the previous night before the night of the event (not the night before the event), getting sick or mentally, not just in the game.

Overtraining is due to getting the body's normal function way out of balance. If you are overtrained, recovery will not occur in a couple of days.

In general, most athletes that race their bikes get a 6 month window of training and performance gains. After six months, a break is needed. That is why many start serious training in February and race through August. They hit August ripping the pedals off the bike and want to keep racing. The reality is that they need a break. If one wants a longer season then a mid summer break is in order.

I suggest taking a couple of days off and if you have not had a rest week in several months, then take a week off and do a couple of recovery rides. Rest is not the enemy.

+1

... the body is FAR more complex than any machine ever dreamt of... And, it has this marvelous property of being able to repair itself far better than we could ever do...

You were on a mtn bike. I assume they were on CX bikes? When you beat them w/out thinking about it, do you also have this significant disadvantage? That could make the physical symptoms seem even worse than they were.