In most cases, issues with Chats can be solved by logging out and logging into your OLX account.
1: To logout, Tap on "My Account" - bottom right of your device
2: Tap on Settings
3: Tap on Logout
4: To log in again, Tap on "My Account" - bottom right of your device
5: Choose a login option and enter your details - you will be logged into your account
On your computer:
1: To logout, at the top right corner next to your profile picture, click on the arrow
2: A drop down list will appear - all the way at the bottom, you'll see a logout option - click on it
3: To log in again, at the top right hand corner, you'll see LOGIN - click on it
4: Choose a login option and enter your details - you will be logged into your account
Have a look at the Chats section and see if your Chat messages have reappeared.
If this doesn't help, then there could possible be something else that's wrong. In that case, please contact us.