Something is wrong here, dead battery...

So, this is the description.

My old charger died recently and I borrowed charger from my friend to recharge battery a little bit before new charger come.
I disconnected main fuse because I have DC-DC converter which use some power all the time. Then, my son came and pushed throttle, wheel rotated few times and everything was dead after that.

New charger came yesterday, everything works while it's connected but there is no power at all after I disconnect it. I doubt on BMS but I have no enough experience to be sure and that is the reason why I ask you guys for help. When ignition is on and charger disconnected voltage drops rapidly and everything is dead after maybe two seconds. Led light on charger is always green and fan doesn't work at all so I think it doesn't charge actually, just maintain voltage while it is connected.

What I wanted to ask is if there is any way to "reset" BMS if it's some kind fooled because of what happened before. System is LiFePO4, 20S.

Thank you very much for your answers and suggestions, with best regards.

Probably it hasn't actually charged, most likely because a cell is so low that BMS is now protecting against charge as well as discharge, but it could be because charger doesn't work (bad connection, wrong charger voltage, etc).

If it hasn't charged, then BMS is still protecting pack.

Check the voltage of each cell group of the battery to find out which one is still so low it prevents output (and/or input) from turning on.

There are hundreds (or more) of battery troubleshooting threads, which will show you the steps needed to do this, and what to do in various problem cases.

Charger is 73V voltage, it can be seen on my instrument when ignition is ON and it is right voltage for my battery pack.
Rear wheel rotates I can turn on the lights, horn, everything but only while charger is connected.

I ordered new BMS and will try to connect it. There is no burn signs on my BMS, though...

My old charger LED light was red and fan worked all the time while charging.

Did you even try checking the cell voltages before ordering parts you probably don't need?

No, I didn't, unfortunately. I can still cancel that order, no problem for that.

Is it possible that some of cells dropped so much after only two seconds before all stopped?

https://endless-sphere.com/forums/viewtopic.php?f=14&t=91145&p=1328434#p1328425

One thing I didn't mention is that initial voltage (when I connected charger for the first time) was around 72. In few minutes it went to what you can see on the picture above.

Maybe I should leave it a little bit longer to charge?

OK, if I accidentally find out that one (or more) of my cells is/are under voltage, may I pass BMS for some time to recharge it?

Below are pictures of my BMS and connections. Could you please help me and tell what to (re)connect and where?
Thank you very much in advance.

Just how old is the battery? And what made you think the charger died? I think its more simple than you think, the battery itself died. that's why the charger refuses to charge it.

On the lower picture, the place you look at cells with a voltmeter is that row of contacts labeled B and a number. on the right, should be your negative. then b1 b2, and so on. I think b1 is your negative. one negative and 20 positives.

As you check cells, the first two contacts is cell one, then cell two is b2 and b3 is cell two, and so on.

I will check and let you know, when I have a chance to do that. Thank you for suggestions.

It's possible one or more cells holds voltage enough to look OK with no load, but drop below the cutoff when there is any significant drain. If all the individual cell voltages look OK, you may need to measure again with some kind of load (light bulb across the meter).