Should one of your Guardian’s lose their phone, your Vault will not be affected and you always have the ability to replace a Guardian at any time.
In general, should a Guardian be out of contact – phone is off, app hasn’t been opened – for an extended period of time, your Vaut12 app will notify you, in fact, you can always see the status of your Guardians at any time by going to the Vault dashboard.
The only scenario in which there could be a risk to your Vault is if the number of active Guardians falls below the minimum threshold needed to unlock assets or restore a Vault. The app will alert you so that you can avoid this circumstance. However, as an overall strategy, you should consider having many Guardians, spread out geographically, and also assign some personal devices as Guardians that you can keep securely. This way, you always have enough Guardians to handle unlock requests and Vault recoveries and to replace any Guardians as needed.