One must be certain of having attained purity. Once purity is acquired either by making Wudhu, Ghusl or Tayammum then it will not be invalidated by mere doubt of impurity. The principle of Fiqh is that اليقين لا يزول بالشك i.e certainty does not become ineffective on the basis of mere doubt.
Thus, if you had Wudhu and you do not remember any reason for Wudhu to become invalid then you will be considered in state of purity. Hence you will be able to offer Salah in such a condition. If you remember any reason to have lost your Wudhu or state of purity then your Wudhu will be considered invalid.
Nevertheless, the safest practice under such circumstances would be to make fresh Wudhu.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America