#include <iostream.h>
#include <iostream>
int main()
{
    using namespace std ;
    const int c=60000;
    long long a[c],i,j,n,ans;
    a[0]=0;
    cin>>n;
    for (i=1;i<n+1;i++){cin>>a[i];a[i]=a[i-1]+a[i];}
    ans=0;
    for (i=1;i<n+1;i++){for (j=i;j<n+1;j++){if (a[j]-a[i-1]>ans){ans=a[j]-a[i-1];}}}
    cout<<ans;
    return 0;
}

Use __int64 instead of long long. And read FAQ, there is a lot of different useful information about avoiding compilation error

Here is your compiling code:
#include <iostream>
using namespace std;
int main()
{
using namespace std ;
const int c=60000;
long long a[c],i,j,n,ans;
a[0]=0;
cin>>n;
for (i=1;i<n+1;i++){cin>>a[i];a[i]=a[i-1]+a[i];}
ans=0;
for (i=1;i<n+1;i++){for (j=i;j<n+1;j++){if (a[j]-a[i-1]>ans){ans=a[j]-a[i-1];}}}
cout<<ans;
return 0;
}