You've not told us whether you're using a 'proper' email client (such as Outlook Express, Windows Live Mail or Thunderbird), an app or web-based access to your mail. You've also not told us whether you're using IMAP or POP for your incoming mail. It might make a difference!
However I've yet to come across a system that doesn't have a 'Trash' folder (which might also be called 'Deleted items', or similar) somewhere. When you delete emails from your Inbox (or from any other folder) they should simply go into 'Trash', where they'll stay until either you delete them from there as well or until a pre-determined time limit is reached. (With the web-based system which I use, the default time is just one day but that can be changed if I so choose).
So you need to look for the relevant folder, to see if your deleted mail is in there. If it is, you can click on an email to select it and then click on 'Move' to transfer it to any other folder (including back into your Inbox).
If you can't see a 'Trash' (or similar) folder with an app or email client, try logging into your email provider's web-based access to look for it there.