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?
Do you have chat logs / emails confirming this (no need to share)?
Did you see the 5 btc arrive on 1M2s5jnkspK82fyiJ1stzG8SDdPhuYcbKP in your wallet?
Did you see the 5 btc arrive on 1M2s5jnkspK82fyiJ1stzG8SDdPhuYcbKP on blockchain.info or something alike?
Did you write down the seed when generating your wallet (before making this transaction)?
Do you have the transaction confirmation hash?
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.
Are you using windows?
Did you download electrum from the official site?
Did you add addresses manually?
No more Yes/No questions, but still important:
Do you use standalone, installer or portable version of electrum?
Which version?
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.
Edit: did electrum generate new addresses when you reopened the wallet file and the address disappeared?
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.