* There may be a mechanical failure or just an obstruction at the latch that holds the leaver down.
* Sometimes the toaster has safety features in-place that test there is live mains voltage before it will latch the lever
** Try replacing the fuse
A rough idea of how this is ment to work;
# push leaver down
# latch grabs it and holds it down, connecting the heating wire mesh to live voltage
# a [link|https://www.electrical4u.com/bimetallic-strip-thermometer/|bimetallic strip|new_window=true] will slowly deform as it heats up
# once fully heated it will be deformed enough to let the latch slip and release the leaver, popping back up and disconnecting the heating element
If you are not comfortable working with live mains voltage, (''and only qualified electricians should be)'' I would not recommend taking your toaster apart or modifying it.