Damaged CMOS-transistor behavior in th IC circuit
Answering this question is akin to opening Pandora's box.
In general a CMOS transistor will be ruined by over-current or over-voltage by breakdown of the gate oxide. Normally, the gate should be isolated from the channel/drain/source, but after breakdown a low-resistive path can be created which hinders the normal operation.
From what I can find, it appears that injection of carriers across the barrier (Fowler-Nordheim tunneling) and the creation of traps/states throughout the oxide layer lie at the base. At some point there seems to be a positive feedback effect that accelerates this trap creation, which increases current, which accelerates trap creation, etc. The end result of this process would be more or less a short-circuit from the gate to semiconductor. Due to the rapid heating, the gate can even be physically destroyed.
How the transistor fails can also vary pretty wildly. There are cases where it doesn't completely destroy the transistor:

Chih-Hang Tung, K. -. Pey, L. J. Tang, Y. Cao, M. K. Radhakrishnan and W. H. Lin, &quot;Fundamental narrow MOSFET gate dielectric breakdown behaviors and their impacts on device performance,&quot; in IEEE Transactions on Electron Devices, vol. 52, no. 4, pp. 473-483, April 2005, DOI: 10.1109/TED.2005.844763.

If you're serious about figuring this out, you'll probably need to start analyzing literature.

J.F. Verweij and J.H. Klootwijk, &quot;Dielectric breakdown I: A review of oxide breakdown&quot;
Microelectronics Journal, Volume 27, Issue 7, 1996, Pages 611-622, DOI: 10.1016/0026-2692(95)00104-2


B. Peŝić, S. Dimitrijev, N. Stojadinović, &quot;Investigation of gate oxide breakdown in CMOS integrated circuits&quot;, Microelectronics Journal, Volume 20, Issue 6, 1989, Pages 19-26, DOI: 10.1016/0026-2692(89)90064-5.