When logging into MyMonero on a new device or after your wallet has been inactive for some time, you will usually have to wait for it to load all of your transaction and blockchain data. You will need to keep your wallet open so that it can scan up to date, this will usually take around 1 hour.
However, if this issue persists for more than a few hours, even after keeping the wallet open, then it usually means that we have a large number of inactive accounts accessing our wallet service all at the same time. This causes delays in our networks ability to scan wallets speedily.
While it temporarily impacts your balance and transactions, it will clear.
Our technical team jump onto these issues immediately, so you can be assured this it will be fixed soon and your funds and transactions will reflect correctly again.