On Samsung phone I meant to click on an email message but instead I swiped it and it disappeared. How can I undo this? It didn't go into the Bin/Trash or Spam.
Click on All Mail on the left. When you see the message, open it, and choose the Move to Inbox option to "un-archive" it.
Just to add, the 'All Mail' system label on the left is not visible by default. To show it, click on the cog button on the top right > Settings > Labels > Show (next to the All Mail system label).
Alternatively, you can use the search feature to locate the message (the search box should be towards the top of the Gmail page).