Well, there are mainly 2 types of help you can get here:
- paying help: in this case, make sure you pick a helper you can trust... It's not wise to send somebody a wallet.dat, but if you really can't figure this one out yourself, sometimes it's the only way
- free help: in this case you'll get some pointers from other members, but you'll have to recover the wallet yourself
Now, let's get things started: did you copy the wallet.dat onto at least 2 external devices (usbsticks for example?). You'll need a backup in case things go wrong!
Next step would be to identify the problem: Did you forget your password, are you 100% sure it's an electrum wallet, do you remember the version or the timeframe? Which error messages do you get? which steps did you already try?
The thing is, for as long as i can remember (not so long, i have a bad memory), electrum's default wallet was called default_wallet, wallet.dat is usually associated to bitcoin core... Now, it's possible older electrum versions did use wallet.dat as default wallet filename, or it's possible you have renamed the default wallet... But i just wonder if you're really 100% sure you're using the correct wallet here?
If the problem is using the wrong wallet, you can either install bitcoin core and see if core opens your wallet.dat, or you can use pyrecover... If the problem is a forgotten password, try out
https://github.com/gurnec/btcrecover