Laptop keeps turning off

Hello,

my Macbook Air behaves strangely. When I turn it on, it goes off. Sometimes I am able to log in, but it turns off in a while, like in less than a minute.
When pressing & holding (the whole time) the left CMD button, this does not happen. Once I release the CMD button, the laptop turns off after a while (in less than a minute again).

Any idea what could cause that? :/

Thank you,

David