as to why, I can't answer - something has changed it.
ok, so you need to create a new robots.txt and upload it to overwrite your existing one.
To do this you'll need to
1. create the file
2. use FTP to upload
1. Use notepad (you'll find it in accessories) and just copy this into it
Save this file as robots.txt to your documents area (or any other area where you can find it again
That's the easy part.
2. To use FTP you need
a) An FTP programme to use to upload.
b) the access details - the host, username and password of your FTP account
It may be that you host provider gives you the programme to allow FTP - go into your host account and see if it is there or discuss with you host provider. If so then you can just upload it - you'll find the existing robots.txt in the root of you website files - just look around for it, and then upload and overwrite it with the new one you've created.
If not you'll need to download one. One of the most used is filezilla client.
Watch this video
start by going as far as 2.30 mins (that'll get you the install)
Then you'll need you host details and you'll use this instead of the "local" he uses in the rest of the video.
b) access details
Your web provider account should have these details - basically you're looking for "FTP account" or similar. If you can't find this, talk to your host provider - they will know what to tell you, just ask for your FTP account host, username and password
Armed with this you can watch the remainder of the video, and learn how to move a file to your server. You'll find the existing robots.txt in the root of you website files - just look around for it, and then upload and overwrite it with the new one you've created.
All the above looks daunting - but take it slowly and you'll find it's quite easy !