You can drill it out using e.g. a cordless drill with variable speed, or a pin vise.
I use a 1.5 mm drill bit.
Pin vise:
Make sure not to put pressure on the glue bottle during drilling, or the glue will come gushing out.
Clean the drill bit immediately after use to prevent the glue from drying on the bit.
If you do get dried glue on the drill bit, you can scrape it off with a knife.
lifehacks.stackexchange.com/users/5307/Hobbes
My usual technique is to use squeeze the nozzle with a pliers a few times to loosen the dried glue, then have a good poke around with needle or pin.
lifehacks.stackexchange.com/users/14071/leigen_zero
Part marked as 'citation needed'
however I just discovered this on Wikipedia doing a totally unrelated search.
Three potential workarounds, as prevention rather than cure...
The reaction with moisture can cause a container of glue which has been opened and resealed to become unusable more quickly than if never opened.
To minimise this reduction in shelf life cyanoacrylate, once opened, can be stored in an airtight container with a package of silica gel desiccant.
Another technique is to insert a hypodermic needle into the opening of a tube.
After using the glue, residual glue soon clogs the needle, keeping moisture out.
The clog is removed by heating the needle (e.g. with a lighter) before use.[citation needed].
The polymerisation is also temperature-dependant: storage at zero degrees or below stops it, so keeping it in the freezer is also effective.
Source:
Wikipedia : Cyanoacrylate
lifehacks.stackexchange.com/users/13777/Tetsujin
Depending on the size of the head opening, i usually find a sharp stanley knife will allow you to cut out the solid glue enough
so you can use the contents again.
But yea
if its really bad, get your smallest drill bit out,
sometimes even a mini screwdriver will do the trick and is quicker then setting up a drill.
lifehacks.stackexchange.com/users/14133/MikeDub
You could also use a pin or similar-sized pointy object and heat it up (ideally with a lighter) and push the hot tip through the dried glue/nozzle.
This has helped me on more than one occasion with minimal damage and spills.
lifehacks.stackexchange.com/users/14194/Jonathan M.
If the glue is based on something called cyanoacrylate (ie crazy glue).
To dissolve or loosen use acetone.
Acetone is commonly used in nail polishes.
You can get the clear kind and dip the end of the device pictured Hobbes post.
Then you're good to go...
lifehacks.stackexchange.com/users/13723/Old_Fossil
According to the package instructions, nail polish remover can be used to remove Krazy glue residue.
In my experience, once open, the Krazy glue won't last very long for a second application.
I have been getting "single application" sizes of the stuff.
lifehacks.stackexchange.com/users/4698/Stan
I have a super glue bottle. The glue inside it is fine, but it has frozen in the nozzle part, that makes it useless.  How to fix this ?  Thanks
Unfreeze super glue stuck in bottle nozzle
lifehacks.stackexchange.com
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lifehacks.stackexchange.com/questions/12097
lifehacks.stackexchange.com/users/14081/Viv
There are a few ways you can try to remove set glue from the nozzle, and possibly the most effective will be a cordless drill with a small drill bit. You could also consider loosening the glue with pliers and then unblocking the nozzle with a pin. Other techniques include suing a stanley knife or dissolving the glue with acetone or nail polish remover. The options are to drill out the glue, to loosen it by squeezing with pliers and then poking it out, to cut it out with a Stanley knife and poke through with a mini-screwdriver, to heat a pin and push it through the glue, to soften it with acetone or nail varnish and to consider purchasing only single use tubes in the future.
Drill it out, avoiding putting pressure on the bottle. Squeeze the nozzle with pliers to loosen the glue then poke it out with a needle or a pin. Cut out the solidified glue with a Stanley knife or poke with a mini screwdriver. Heat up a pin and push through the solidified glue. Use acetone to loosen the glue. Try nail polish to soften or just buy small 'single application' tubes of the glue.