Okay. Your answer is still not 100 pct clear to me.
Please answer with Yes / No to each question.

Did you request a payment to 1M2s5jnkspK82fyiJ1stzG8SDdPhuYcbKP?  if by requesting you mean request from electrum, no. but i copied the address and email the guy
Do you have chat logs / emails confirming this (no need to share)? yes sure, can even make the guy confirm by signing his address, no problem
Did you see the 5 btc arrive on 1M2s5jnkspK82fyiJ1stzG8SDdPhuYcbKP in your wallet? yes
Did you see the 5 btc arrive on 1M2s5jnkspK82fyiJ1stzG8SDdPhuYcbKP on blockchain.info or something alike? yes, its there
Did you write down the seed when generating your wallet (before making this transaction)? yup i have the seed
Do you have the transaction confirmation hash?  not understanding, if the guy who sent need to find something in the transaction, i can ask him, he will provide no problem.
It looks like this

The seed you have now, does it produce the master pub key you shared in this thread?
It should start with the address 1M2WeprC3o6P7YvLvXEAFZBQVFxs4iPqKD.
I checked by making a watch-only wallet with your key.   can you be more step by step wise?

Are you using windows? yes
Did you download electrum from the official site? yes
Did you add addresses manually? no generated when i installed

No more Yes/No questions, but still important:
Do you use standalone, installer or portable version of electrum? installer or standalone but definitly not portable
Which version? 2.3.2


If you are using standard windows 7, go to your c:\users\FILL_IN_YOUR_USERNAME\AppData\Roaming\Electrum folder.
Tell us which files you see there.
It contains your settings and your wallet files.   i will send that when i return home from work, will check and say what files there are.


Edit: did electrum generate new addresses when you reopened the wallet file and the address disappeared?  no the address the 5btc was sent to just dissapeared. the rest of the addresses stayed the same.
The odds are low for 2 consequent seeds to have their first address to start with 1M2... this smells like an auto-complete typo on your end.  no was not a typo mistake, i copied and pasted. not write it myself, its too long and complete to write those addresses.