I have Dbase PLUS 10, it is quite old, but before today it worked reliable, but several days ago at opening .dbf file I got: Not a valid dBASE table. I need to repair dbf, any suggestions...

I would check the dBase Plus 10 newsgroup
 
http://news.dbase.com/newsgroups.php

Sounds like the header got corrupted or the file got truncated.
If it's header corruption you might be able to repair the file or at least extract some of the data.
 
#1 rule on fixing DBF files - always run fix on a copy.
Running a repair or fix on your only copy may make it unsalvageable.
 
Years back there used to be a lot of DBF repairs tools available, but few were free.
A google search should turn up a number of them.
Here's 2 I came across, they both have free trials - don't know what the limitations of the trial versions are:
http://www.dbase.fixdbffile.com/ 
 
http://www.dbfrepairtools.com/?gclid=CjwKEAjwv7HABRCSxfrjkJPnrWgSJAA45qA2Jaflgq9698OGdovvT3p3qJSqkmlBvSzby-x_q-7JchoCVUDw_wcB
 

More info:
DBFs are essentially text files - depending on what you need to get out of the database you might be able to open file in a text editor or hex editor and locate what you need.
 
If you're going to try this, try it on a copy. If you accidentally save the file in the text editor you may end up corrupting it further.
 
BTW Note fields are not saved in the DBF but in linked DBT or other file.
The order of the notes does not match the order of the records in the DBF. 
 

I remember moving to Clipper back in the mid 80s.

Yes, repair a copy, not the original.
 
If memory serves, FoxPro (for DOS) seems to be more forgiven in opening invalid DBF. If you can get hold of FoxPro, open it (the copy) and run a "pack" command usually solves the problem.

Wow, a lot of tips, many thanks for articles and solutions, I'm sure one of these suggestions must assist.

MS Excel can import/export .dbf files too.

Kev999
MS Excel can import/export .dbf files too.

Open/import maybe - export, not really.
Don't open a DBF in Excel and then save it..
Excel bases field size on the spreadsheet column width, minimum width being the field name which maxes at 11 characters.
So you can instantly loose a lot of data and end up with all character fields. 
Plus there are multiple versions of and flavors of DBFs, I think Excel saves them as dBase v3.1.
 

Worth trying also by setting up ODBC connection to the DBF file as the support is built right into Windows. Maybe at least you can read the data from the DBF file, if there is no error connecting it.