Entry from OED Online

    21. square one: the beginning, the starting-point. Freq. as back to (also in, on) square one.
  Often said to derive from the notional division of an association football pitch into eight numbered sections for the purposes of early radio commentaries (see Radio Times, 1927, 28 Jan.). This suggestion cannot be upheld with any certainty, and the phrase may simply come from a board-game such as Snakes and Ladders.
 
  1960 Times 21 May 9/2 As far as building up a basis for profitable negotiations is concerned the two sides are back in square one. 1965 Listener 24 June 930/2 Let us drop the logical knot that twin studies have tied us in and go back for a moment to square one. 1965 Guardian 13 Oct. 2/7 The city's medical officer..said they were still in ‘square one’, and would stay there till they got some real facts. 1966 J. I. M. STEWART Aylwins x. 126 That he had seized a chance to break off our interview at that point seemed to argue a refusal to abide by this judgement of the matter. We were back, so to speak, in Square One. 1970 G. F. NEWMAN Sir, You Bastard 279 A couple of wrong answers and Sneed knew he'd be right back on square one. 1973 G. TALBOT Ten Seconds from Now (1974) viii. 111 After each of those successful essays it was ‘back to Square One’. 1977 ‘M. INNES’ Honeybath's Haven x. 98 Honeybath broke off in these bold proposals, suddenly aware that Edwin was weeping. It was like being back on square one. 1980 D. BOGARDE Gentle Occupation xii. 332 ‘Black is black, Moluccans.’..‘Are coloured people. They are dark,’ said Emmie with force. ‘Well, don't let's have any blasted children’... ‘But I do. I want.’..‘Oh for God's sake. We're back to square one again.’