I remember a similar instance where the server was out of RAM. 
If you notice that nothing is happening and your website is still functioning properly, you may want to click the stop button. If the upgrade did not go through, you can rebuild it from an SSH console.
cd /var/discourse
./launcher rebuild app
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Yes, you are right @Arkshine.
I recommend you first check you have set swap before carrying on!
(Discourse will not build on a 2GB machine reliably without swap these days)
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Not even on 4GB without swap.
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