No problem – this sort of thing happens all the time. In your Migros account, there is a link labelled "Forgotten your password?". You can reset your password by clicking on the link. Next, check your e-mail inbox and your spam folder, and follow the instructions in the e-mail you receive.
The procedure is exactly the same way for your Cumulus account. If you need help, the Cumulus Infoline will be happy to assist you.