Reset manual for HPE 1820-24g, 1820-48g.
HP says that you hit both buttons at the same time for 10 seconds.
That link os for an 1810 model but the instructions will probably work for your 1820.
HPE_c04622696_HPE OfficeConnect 1820 Switch Series Installation and Getting Started Guide
Found that link for the 1820 and it does have different instructions.
The Reset button is used to restore Factory Default settings, or reset the switch while it is powered on.
Restoring Factory Default Configuration — Press and hold down for over 5 seconds, the switch will then complete its self test and begin operating with its configuration restored to the factory default settings.
Any configuration changes you may have made through the web browser interface are removed.
If a DHCP service is not available in your network, or for some reason the switch does not acquire an IP address from the service, the switch defaults to IP address 192.168.1.1 after 120 seconds of automatically attempting to acquire an IP address.
Figure out what IP number the switch is on and use a browser to login to the switch
The document says the default login as admin with no password.