Hi Linlyn,
Welcome to Apple Support Communities. We understand you accidentally selected, "Forget This Network" on an iPad, and you'd like to rejoin the same Wi-Fi network.
Establishing a W-Fi connection is important to complete your daily tasks. We'll be happy to help.
If the Wi-Fi network has a password, you'll need to re-enter the password for that network on your iPad. If there's no issue with the password, we suggest to follow the recommended steps outlined in this helpful article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
Best regards.