They probably will. Technically you should have it replaced with a single entry visa before trying to reenter. But several people have been okay explaining their situation and been allowed in. The Home Office may want you to report to them within x number of days showing that you are legal, but that is unlikely if they are able to verify your info.
You will need to get it replaced when you return to the UK, which will take a few months and some ££. Are you absolutely sure it's lost?