Synchronization stops for a long time.
Daedalus wallet is a full node wallet. This means it creates a copy of the entire Cardano blockchain on your machine. Sometimes this blockchain data becomes corrupted causing Daedalus wallet to stop working properly. Deleting the blockchain and syncing it again is a solution for some types of problems encountered while using Daedalus.
You need to delete and restore blockchain data.
See other Known Issues