i would use blunt ended instrument and not tip of kitchen knife!!!Normal:
View attachment 202297
Cap removed and piston available for push:
View attachment 202298
I was able to remove the cap with my fingernails. Basic kitchen knife should work if you don't have as girly fingernails as me, lol
If you have emptied the tube to some degree, then you need something to reach the piston inside of course. The same kitchen knife or what ever stick should do the trick.
but very good info @KvanH