Portal burnout in effect.
Burnout occurs when a Portal has been hacked by an Agent four times within four hours. (Not to be confused with Cooldown, which is typically 5 minutes) Burnout lasts for four hours after the first hack, preventing the Agent from hacking the Portal within that time. When an Agent attempts to hack a burnt out Portal, an alert will pop up, reading "Portal burned out! It may take significant time for the Portal to reset."
Burnout can be delayed using Multi-Hacks 
, which increase the number of hacks an Agent can make before the Portal burns out. Installing a Heat Sink 
will immediately reset the burnout counter for the Agent who installed it.
Like cooldown, burnout is per Agent; hacking never affects other Agents' gameplay. Other Agents can still hack a Portal that burnt out for you.
Agents will also get the message if they try to Drone Hack the same Portal twice in a row. Note that if a Portal is burned out from physical hacks, the burnout also applies when trying to hack via Drone, and vise-versa.