Have you blocked a user by mistake or chosen to temporarily block them on Twitter? You can find the list of users that you have blocked on Twitter and then unblock them, regardless of how you access your Twitter account. Once you unblock a user, you can follow them again so that you can receive their updates.
Steps
Method 1 of 3: From the Twitter site

Step 1. Click on your profile picture in the upper right corner and select Settings
You will access your account settings.

Step 2. Click the Blocked Accounts option in the menu on the left
A list of all the accounts you have blocked will be displayed.

Step 3. Click the Blocked button next to the account you want to unblock
When you hover the cursor over the button it will display the option Unblock

Step 4. Click the Follow button to start following this user again
The button Blocked will turn into To follow after unblocking the user. Click on this button, if you want to follow the user again.
Method 2 of 3: Using the Twitter app (iOS)

Step 1. Tap the Me tab at the bottom of the screen
This will display your profile.

Step 2. Press the gear button near your profile picture and select Settings
This will open your account settings.

Step 3. Select Privacy and Content
You will access your privacy settings.

Step 4. Scroll down and tap Blocked Accounts
This option can be found in the section Contents.

Step 5. Tap the Blocked button next to the user you want to unblock
This will unblock the person.

Step 6. Navigate to the user's profile and tap Follow to follow this user again
Select the unblocked user from the list of blocked users to access their profile. Push the button To follow to start following that user again.
Method 3 of 3: Using the Twitter Android app

Step 1. Press the ⋮ button in the upper right corner and select Settings
This will open your Twitter account settings.

Step 2. Tap Privacy & Content
This will open your account settings.

Step 3. Tap Blocked Accounts at the bottom of the menu
This will display the list of accounts that you have blocked.

Step 4. Tap the Blocked button next to the user you want to unblock
You will be notified that the user has been unblocked.

Step 5. Navigate to the user's profile to start following them again
After unblocking the user, tap their name to access their profile. Push the button To follow to start following that user again.