You did not tell us which laptop you are using. Maybe it supports Heads instead, which would be even better AFAIK.
However I would not recommend to go full-security so quickly for a person new to all this stuff. There many other attacks that are much more frequent than Evil Maid, and it takes time to adjust your workflow to Qubes OS with many isolated workflows.
In this case, if your computer was doing some writing to the disk, the writing fill fail and the file will be damaged. D epending on which file it was, you may loose some data or render your OS unbootable. Nobody should be able to deanonymize you because of that. It’s not recommended to abruptly shut down Qubes OS, but usually it does not do any harm.