If the yanked text is not too big, you should take a look at the output of :reg
. What you yanked is probably in register 1 or 2.
Registers are saved between sessions but there's a hard limit on the size of their content. You can see that limit on your system with :set viminfo?
: the default value should include <50
and s10
meaning that the content of registers is saved if they consist of less than 50 lines and/or weight under 10 Kb.
If you yanked a couple of reasonably lengthy lines, you should be fine.
See :h 'viminfo'
if you want to change these value.