The flashing white led generally means the Sonos product is booting up after being plugged into power.
I’d therefore check the power connection is fully inserted into the Arc. It should sit flush and not be in any way loose.
I would also try with the Sub also wireless. The Arc will use its 5GHz radio to communicate with Sub/surrounds.
And, if neither of those achieve the desired affect, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.