MX records determine which service provider you are using for your emails, and where to deliver emails to.
So if you set MX records different from the Hostinger default ones, you won't be receiving any new emails to your Hostinger email accounts. However, you will be able to create new email accounts and send emails.
In case you want to set Hostinger MX records again, it can be easily done via hPanel.
Applying new MX records takes up to 24 hours