I opened one of my Electrum wallets to find that it had no transactions listed and no coins.
I was calm.
I can't say how much Bitcoin I believed had been in this wallet (for obvious reasons). Let it suffice that it was a significant amount that I could not easily replace.
I was calm.
I checked my other wallets. I checked old wallets that I had abandoned (but had kept the files around for record keeping purposes). I checked Electrum's default wallet.
Nothing.
I started to lose my calm.
But as I began frantically opening and closing Electrum wallets, it eventually got confused enough that it had to open the REAL default wallet (the "default_wallet" I had already checked actually belonged to an older version of Electrum, and I hadn't realized that).
And there it was. When I had made my last transaction, I ended up sending it to Electrum's default wallet instead of the wallet I had set up specifically for that amount of Bitcoin.
But it was too late to calm down. My heart is still fluttering with the thought of what I almost lost. In this state, I was reminded of the parable of the woman and the piece of silver. So I decided to come here and declare my good fortune.
I have since transferred the private key containing that amount of Bitcoin to one of my known wallets, and then deleted the default wallet.
Celebrate with me, for that which was lost is found.